1. Field of the Invention
The present invention relates to a cable television/telephone tap, and more particularly to a cable television/telephone tap for transmitting TV high-frequency signals to subscribers from a coaxial cable TV signal transmission system installed in general homes, communal housing, offices and the like, while simultaneously supplying low-frequency current to telephones, either in a multiplexed format on the coaxial cables, or by means of current-splitting wires provided separately from the coaxial cables.
2. Description of the Related Art
A typical example of a conventional device for directing high-frequency signals from a coaxial cable signal transmission system to electronic apparatuses is a tap-off. Tap-offs are widely used in cable television or joint TV viewing facilities in communal housing. This type of system and device will next be explained.
FIG. 9 is a schematic diagram of a CATV communal reception facility utilizing a conventional TV signal tap. In FIG. 9, 101, 102, 103 and 104 are groups of antennas for receiving TV signals, wherein 101 and 102 are parabola antennas for CS reception at superhigh frequency, 103 is an antenna for ground wave reception at very high frequency, and 104 is an antenna for ground wave reception at ultra-high frequency.
All TV signals received by these antenna groups are led to a mixing amplifier 121 by means of coaxial cables 105, 106, 107 and 108. In addition, in a CATV viewing reception facility 120, video-taped programs from VTRs 122, 123 and 124 are level-adjusted and shaped by a mixer 125 and then led along coaxial cables to the input of the mixing amplifier 121. After output has been set at a predetermined level, the video signals are input to a trunk amplifier 131.
Furthermore, a branch amplifier 132 comprises four splitters, but here CATV signals split into four equal portions are fed through coaxial cables, to which taps have been connected in multiple levels, and are transmitted to all TV viewers by tap-offs 133 and 134.
In this case, all the antennas, mixers, amplifiers, VTRs, channel-changers, monitors and the like are effectively united in a single system, in the nature of a head terminal. The term "headend" customarily denotes all these apparatuses inclusively.
Electrically speaking, a conventional CATV tap-off is a device which uses a directional coupler to couple one portion of a high-frequency signal, which has been input to the input terminal, to a branch terminal, and outputs the remaining portion of the signal directly. This constitutes a hybrid circuit, wherein high-frequency signal entering from the output terminal side are sent to the input side without being coupled to the branch terminal, in order to prevent the admission of noise generated by TVs which are positioned further down the communications line than the tap-off. The usual frequency band is approximately 5-1,000 MHz.
FIG. 10 is a schematic view of a configuration of a conventional CATV tap. As FIG. 10 shows, an input terminal 151, to which the inner conductor of a coaxial cable is connected, leads high-frequency signals inside an enclosure. The inner conductor is insulated by means of a metallic case 152 comprising upper and lower parts which are linked together. The external conductor of the coaxial cable is connected to the enclosure on the input side by the input terminal 151 and a coaxial cable connector shell 154. An output terminal, having a similar configuration to the input terminal 151, is provided on the opposite side of the enclosure to the input terminal 151.
Branch terminals 153 are extracted by connectors on the surface of the lower part of the enclosure 152, with the inner conductor still in an insulated state. These are connected in an exposed state to messenger wire strung between utility-line poles along roads and the like, generally by screwing the strand wire into a strand clamp 155, which has a screw-hole of specified dimensions.
Such conventional taps are only capable of splitting signals within a frequency band of approximately 5-1,000 MHz, and cannot be used for CATV telephone services, which are expected to proliferate in the near future.
However, as FIG. 11 shows, since power current from an amplifier flows through the trunk line which is connected to the input/output terminal I/O, a method is envisaged in which, in addition to branching high-frequency signals by means of a directional coupler DC and a splitter SP, choke coils L are used to divert current from the trunk line, and this current is then multiplexed with the high-frequency signals and split-fed to user terminal B. Some of the problems affecting this type of system are listed below.
1. Approximately 200 mA of current must be supplied to each subscriber, but branch terminals in conventional taps suffer from unreliable core contacts when used over a long period. PA1 3. Devices at the subscriber end require an apparatus capable of separating the high-frequency signals from the low-frequency current for telephones. PA1 4. When a subscriber wishes to cancel his telephone subscription while continuing to subscribe to the TV reception service, it may be necessary to temporarily cut off the telephone and TV services of other subscribers as well in order to implement this. PA1 5. With the additional devices mentioned in points 2, 3 and 4 above, the side of the tap is noticeably greater than conventional taps, making handling of the tap inconvenient.
2. Since choke coils L, the number of which corresponds to the number of subscribers, are inserted on the trunk line in parallel, the high-frequency transmission properties of the trunk line may be adversely affected by resonance caused by coupling between the choke coils L.